1. Field of the Invention
This invention relates to an improved wye-delta open transition motor starter with leading phase monitor, and the method for using the leading phase monitor with the starter.
2. Background Information
Wye-delta motor starters have long been used for starting three-phase motors. The use of such starters results in the application of a reduced voltage to the motor windings during starting. By connecting the delta windings of the motor in wye for starting, the voltage on the motor windings is reduced by 33.3% and the starting current and torque by 66.6%. Once the motor reaches or approaches its operating speed, the wye connection is opened and a delta connection is made.
Since a direct short circuit would result if the delta connection of the windings was made before the wye connection was opened, a short delay is designed into the starter control circuit. This delay is customarily in the order of 2-4 line cycles or enough time for the motor starter wye contactor to drop out before the delta contactor closes. Such a delay solves the short circuit problem by momentarily leaving the windings in an open circuit condition. This mode of wye-delta starting is commonly referred to as "open transition" starting. Open transition starting is not commonly used because of the very large current transients which can occur when the delta contactor is closed. The peaks of such current transients can be over 23 times the full load current of the motor. These transients are primarily attributable to the phase displacement of the voltage induced in the motor winding from the line voltage to which the motor winding is to be connected.
As a result of the recognition of this transient current problem, another form of wye-delta starting called "closed transition" was developed. A "closed transition" starter connects a set of three resistors in series with the motor windings to the power lines to keep the motor winding voltages in phase with the power lines during the transition period. This solves the transient problem, but adds considerable bulk and expense to the starter.
Still another form of a wye-delta motor starter has been disclosed which is intended to overcome the current transient problems of an open transition wye-delta starter. U.S. Pat. No. 3,624,471 issued Nov. 30, 1971 sets forth a wye-delta starter having contactors which are progressively and in a timed sequence actuated to change the connection of the motor windings from wye to delta by varying the winding connections in such a way that at least one winding is energized at all times. Thus, the only known alternatives to "open transition" starting, which introduces high transient currents, are those providing a closed or a modified closed transition, both of which alternatives add circuit elements which are bulky and expensive.